Convert symbolic relations to/from SymPy

[rwst]

sage: (x>0)._sympy_()
...
NotImplementedError: relation

Reversely in SymPy:

In [9]: x >= 0
Out[9]: x ≥ 0

In [10]: _._sage_()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-10-7149a2430cab> in <module>()
----> 1 _._sage_()

AttributeError: 'GreaterThan' object has no attribute '_sage_'

In [11]: Eq(x, 3)
Out[11]: x = 3

In [12]: _._sage_()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-12-7149a2430cab> in <module>()
----> 1 _._sage_()

AttributeError: 'Equality' object has no attribute '_sage_'

This functionality is needed both by #22322 and #23923. Needed is a fix to SympyConverter in expression-conversions.pyx

The reverse patch in SymPy is added here, see the PR sympy/sympy#13420

CC: @mforets

Component: interfaces

Author: Ralf Stephan

Branch/Commit: 479e206

Reviewer: Emmanuel Charpentier

Issue created by migration from https://trac.sagemath.org/ticket/23990

[rwst]

Branch: u/rws/convert_symbolic_relations_to_from__sympy

[rwst]

New commits:

9462954

23990: Convert symbolic relations to SymPy

[rwst]

Commit: 9462954

[rwst]

Description changed:

--- 
+++ 
@@ -5,4 +5,30 @@
 NotImplementedError: relation
 ```
 
+Reversely in SymPy:
+
+```
+In [9]: x >= 0
+Out[9]: x ≥ 0
+
+In [10]: _._sage_()
+---------------------------------------------------------------------------
+AttributeError                            Traceback (most recent call last)
+<ipython-input-10-7149a2430cab> in <module>()
+----> 1 _._sage_()
+
+AttributeError: 'GreaterThan' object has no attribute '_sage_'
+
+In [11]: Eq(x, 3)
+Out[11]: x = 3
+
+In [12]: _._sage_()
+---------------------------------------------------------------------------
+AttributeError                            Traceback (most recent call last)
+<ipython-input-12-7149a2430cab> in <module>()
+----> 1 _._sage_()
+
+AttributeError: 'Equality' object has no attribute '_sage_'
+```
+
 This functionality is needed both by #22322 and #23923. Needed is a fix to `SympyConverter` in `expression-conversions.pyx`

[sagetrac-git]

Changed commit from 9462954 to 07f4744

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

07f4744

23990: handle unequality

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

0ed659f

23990: do not evaluate

[sagetrac-git]

Changed commit from 07f4744 to 0ed659f

[sagetrac-git]

Changed commit from 0ed659f to 5f023ea

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

5f023ea

23990: convert relations from SymPy to Sage, with test